Color measurement systems for performing color measurements of printed sheets and, possibly, automatic adjustment and/or setting of inking units of a printing press are already known as such in the field of commercial printing. Such known systems are typically used in connection with commercial offset printing presses that are used to print various types of commercial products using the well-known four-color CMYK (Cyan-Magenta-Yellow-Key Black) subtractive color model, i.e. by printing multicolor patterns consisting of a combination of halftone raster patterns printed using the four primary colors Cyan, Magenta, Yellow and Black.
International application No. WO 2007/110317 A1 (and corresponding US publication No. US 2010/0116164 A1), which is incorporated herein by reference in its entirety, for instance discloses a method for adjusting an inking unit of a printing press. During a setup phase of the printing press, a small number of sheets are run through the printing press and the resulting printed sheets are inspected by means of a first measuring device (which is not integrated into the printing press), such as a densitometer, color spectrometer or a measuring instrument for combined densitometric and colorimetric measurements. The values measured by the first measuring device are compared to predetermined reference values and adjustments of the inking units of the printing press are made so that the values measured by the first measuring device match as closely as possible the desired reference values. A set of “first actual values” representative of the desired settings are thereby determined and stored as a result of the setup phase and the printing press can be released for production runs. At least a second measuring device is provided downstream of the printing units of the printing press in order to inspect the sheets during production, which second measuring device is installed in the printing press. Such second measuring device is for instance embodied as an in-line inspection system comprising at least one camera system and at least one illumination unit. The camera system is typically a color camera system comprising a line-scan sensor or an array sensor based on CCD or CMOS technology. The illumination unit typically comprises light-emitting diodes, or LEDs, or like illumination elements. The second measuring device records an image of at least one, preferably of all of the sheets which are printed on the printing press and converts the recorded images to digital image data which is fed to an image processing system as a set of “second actual values”. During a learning phase, the set of “second actual values” is measured and stored as reference values for controlling an adjustment unit which adjusts the inking units of the printing press. Upon completion of the learning phase, all further printed products which are produced on the printing press are evaluated on the basis of the reference values that were established during the learning phase and any deviation between the reference values and the measured values which exceeds an acceptable tolerance is corrected by means of the adjustment unit.
According to WO 2007/110317 A1, measurements are typically made on at least one measuring strip (or “color control strip”) that forms part of the patterns printed on the sheets, which measuring strip is typically located in a margin of the sheet, such as the margin at the leading edge of the sheet, outside the effective printed region of the sheet where the actual prints are carried out.
Known solutions as above are used for performing color measurements in commercial offset printing presses, i.e. printing presses of the type based on four-color composite printing using the CMYK subtractive color model. Printing presses of this type comprises at least four distinct printing towers which are each designed to print one of the four primary colors. Additional printing towers may be provided to print special colors and/or for the purpose of coating the printed substrates.
These solutions are satisfactory as far as applications to commercial offset printing presses are concerned and basically require the use of a rather simple color control strip comprising a plurality of color control fields representative of the relevant primary colors that are printed (i.e. Cyan, Magenta, Yellow, Black) and, possibly, simple combinations thereof (e.g. Blue/Cyan+Magenta, Red/Magenta+Yellow, and Green/Cyan+Yellow) and/or additional special colors.
In contrast to commercial (offset) printing, security printing (as applied for instance for the production of banknotes and like printed securities) is not based on the use of a four-color printing process relying on the CMYK subtractive color model. Rather, solid patterns are printed using different printing inks of the desired colors (i.e. a blue pattern is printed using a blue printing ink, a brownish pattern using a brownish ink, a copper-like pattern using a copper-colored printing ink, etc.).
Typical color control strips as used in commercial printing are not suitable for security printing applications for the purpose of measuring the printed colors, even less for the purpose of automatically controlling the ink supply. There was therefore a need for a new and improved solution which could suitably cope with the specific requirements of security printing. A solution to this problem has been proposed by the present Applicant in European patent application No. 10187099.6 of Oct. 11, 2010 entitled “COLOR CONTROL PATTERN FOR THE OPTICAL MEASUREMENT OF COLORS PRINTED ON A SHEET-LIKE OR WEB-LIKE SUBSTRATE BY MEANS OF A MULTICOLOR PRINTING PRESS AND USES THEREOF” (to be published as European patent application No. EP 2 439 071 A1), which application is incorporated herein by reference in its entirety.
European patent application No. 10187099.6 especially relates to a color control pattern as generally illustrated in FIGS. 2 to 4 of the instant application, namely a color control pattern CP for the optical measurement of colors printed on a sheet S (or web) by means of a multicolor printing press, especially by means of a multicolor security printing press, which sheet S exhibits an effective printed region EF having a multicolor printed image comprising a plurality of juxtaposed colored areas A-H printed with a corresponding plurality of printing inks of different colors, wherein the color control pattern CP is printed in a margin portion lm of the sheet S next to the effective printed region EF. Such color control pattern CP comprises one or more color control strips a-d extending transversely to a direction of transport T of the printed sheet S, each color control strip a-d comprising a plurality of distinct color control fields CF, CFA to CFH consisting of printed fields of each relevant printing ink that is printed in the effective printed region EF. These color control fields CF, CFA to CFH are coordinated to actual usage of the relevant printing inks in the effective printed region EF and are positioned transversely to the direction of transport T of the sheet S at locations corresponding to actual positions where the relevant printing inks are applied in the effective printed region EF.
Preferably, the effective printed region EF consists of a matrix of individual multicolor prints P, especially multicolor security prints, arranged in multiple rows and columns and the color control pattern CP comprises an individual color control pattern CPi, CP1 to CP5 for each column of individual multicolor prints P. All such individual color control patterns CPi, CP1 to CP5 are preferably identical.
Advantageous designs of the color control pattern CP are further discussed in European patent application No. 10187099.6.
European patent application No. 10187099.6 further relates to a color measurement system comprising an optical measurement system for measuring the colors printed on the sheet S, wherein measurement of the printed colors by the optical measurement system is carried out in a color control pattern CP as defined above.
The above-defined color control pattern CP (and color measurement system) can advantageously be used for the purpose of:
(i) performing in-line color measurements in a multicolor printing press, especially in a multicolor security printing press, or for performing offline color measurements (i.e. outside of the multicolor printing press); and, possibly,
(ii) automatically adjusting and/or setting inking units of the multicolor printing press.
Offline inspection and color measurement of printed matter is known as such in the art. In contrast to in-line inspection which is carried out, usually, on each and every sheet processed in a printing press, offline inspection is typically carried out to perform a so-called statistical process control (SPC) of the production, i.e. by taking sample printed sheets out of the production and checking such sample printed sheets to possibly identify deviations in the printing quality. Statistical process control (SPC) is typically carried out in commercial printing as discussed in the above-identified patent publications and generally consists in performing spectrophotometric, colorimetric and/or densitometric measurements in color control strips or fields provided next to the effective printed region of the sheets. Spectrophotometric or colorimetric measurements (i.e. measurements of the spectral distribution of printed colors) are typically carried out by means of a spectrophotometer or spectrocolorimeter. Densitometric measurements (i.e. measurements of the optical density of or degree of light reflected from—the printed colors) are typically carried out by means of a densitometer.
An example of a device for color measurement of printed sheets comprising a densitometer head mounted on an arm which is moveable along an x-axis over a sample printed sheet, which densitometer head is moveable on the arm along a y-axis is disclosed in U.S. Pat. No. 5,208,655.
International application No. WO 02/08730 A1 discloses a device for color measurement of printed sheets comprising a spectrophotometer mounted on a x-y table so as to be able to be positioned at any desired location over a sample printed sheet, wherein a camera is further provided to capture an image of the entire sample printed sheet. The image of the sheet is processed according to a pattern recognition technique in order to identify areas on the sample printed sheet where the spectrophotometer is positioned to perform color measurements.
U.S. Pat. No. 5,163,012 discloses a quality control system for quality control of printed sheets, which quality control system in particular combines a video camera adapted to capture an image of the entire surface of a sample printed sheet and a color measurement device (such as a densitometer or colorimeter) that can be moved manually by an operator over the surface of the sample printed sheet to perform color measurements at desired locations on the sample printed sheet. The color measurement device is provided with a marking that is imaged by the video camera so as to ensure that the appropriate measurement is being taken at the correct place.
Further examples are disclosed in publications DE 10 2007 041 673 A1, DE 295 06 268 U1, DE 10 2007 030 571 A1, GB 2 107 047 A and DE 296 07 898 U1.
The above known solutions are reasonably satisfactory to the extent that they can be adequately used in commercial printing. Such known solutions are however inadequate when it comes to inspecting and performing color measurements on printed sheets as used for the production of banknotes and liked printed securities. In such applications, the operator or printer not only needs to ensure proper printing quality in terms of the colorimetry and density of the visible colors that are applied on the printed sheets, but also needs to ensure high printing quality. In particular, the printer has to ensure that all security features that are to be applied on the printed sheets are properly printed on the sheets, including but not limited to invisible features that are only visible under certain illumination conditions, such as infrared or ultraviolet illumination.
There is therefore a need for an improved solution which enables the printer to adequately check all relevant features that are to be printed on the sheets.